A gas turbine engine typically comprises a multi-stage compressor, which compresses air drawn into the engine to a higher pressure and temperature. A majority of this air passes to the combustors, which mix the compressed heated air with fuel and contain the resulting reaction that generates the hot combustion gases. These gases then pass through a multi-stage turbine, which, in turn drives the compressor, before exiting the engine. A portion of the compressed air from the compressor bypasses the combustors and is used to cool the turbine blades and vanes that are continuously exposed to the hot gases of the combustors. In land-based gas turbines, the turbine is also coupled to a generator for generating electricity.
Turbines are typically comprised of alternating rows of rotating and stationary airfoils. The stationary airfoils, or vanes, direct the flow of hot combustion gases onto the subsequent row of rotating airfoils, or blades, at the proper orientation such as to maximize the output of the turbine. As a result of the hot combustion gases passing through the vanes, the vanes operate at a very high temperature, typically beyond the capability of the material from which they are made. In order to lower the operating temperatures of the vane material to a more acceptable level, vanes are often cooled, either by air or steam. Typically, turbine vanes are configured in multiple segments, with each segment including a plurality of vanes. This configuration is well known in order to minimize hot gas leakage between adjacent vanes, thereby lowering turbine performance. While this configuration is advantageous from a leakage perspective, it has inherent disadvantages as well, including an increased stiffness along the platform that connects the adjacent vanes, relative to a single vane configuration.
A vane assembly 10 of the prior art, is shown in FIG. 1, and comprises an inner platform 11, inner rail 12, outer platform 13, and vanes 14 extending between inner platform 11 and outer platform 13. While the inner rail serves as a means to seal the rim cavity region from cooling air leaking into the hot gas path instead of passing to the designated vanes, inner rail 12 also stiffens inner platform 11. Inner rails 12, which can be rather large in size, are located proximate the plenum of cooling air and are therefore operating at approximately the temperature of the cooling air. As a result, hot combustion gases passing around vanes 14 and between inner platform 11 and outer platform 13 cause the vanes and platforms to operate at an elevated temperature relative to the inner rail. This sharp contrast in operating temperatures creates regions of high thermally induced stresses in vanes 14 and along inner platform 11 that has been known to cause cracking of the vane assembly requiring premature repair or replacement.
What is needed is a vane assembly configuration that lowers the operating stresses in the vane and inner platform for a vane assembly having an inner rail portion that is exposed to lower operating temperatures than the platform or vane.